Antibodies such as monoclonal antibodies are widely studied as binding molecules that recognize and specifically bind to a target. Antibodies are generally produced by immunization to animals. However, it is difficult to produce antibodies against low molecular weight compounds such as ion atoms and peptides or antigens highly conserved in biological species, for example. Therefore, it is not always possible that an antibody that specifically binds to a target can be obtained. Thus, in recent years, a nucleic acid molecule such as an RNA oligonucleotide or a DNA oligonucleotide, being capable of specifically binding to a target have received attention as a substitute for an antibody. The nucleic acid molecule is generally called an aptamer. It has been reported that an aptamer that specifically binds to a target such as a low molecular weight compound from which it is difficult to obtain an antibody, can be obtained (Non-Patent Document 1).
As described above, it is possible to obtain an aptamer that specifically binds to a target from which it is difficult to produce an antibody. Therefore, for example, using the aptamer as an important tool in the biochemical field and the medical field are attempted as described below.
For example, it is possible to chemically synthesize a large quantity of aptamer. Some aptamers have low immunogenicity and show a strong binding ability to a target as compared with antibodies. Therefore, aptamers can be candidates of superior molecular target drugs (Non-Patent Document 2). Specifically, Pegaptanib (generic name: Pegaptanib, product name: Macugen) is known as an aptamer that binds to a vascular endothelial growth factor (VEGF). This aptamer has been approved as a therapeutic drug for age-related macular degeneration in the United States, Europe, and Japan (Non-Patent Document 3). In addition, currently, clinical trials of at least five kinds of aptamers are conducted in the United States.
Moreover, studies to utilize aptamers as novel molecular sensors are conducted actively. For example, an aptamer against a target such as a serum protein (Non-Patent Document 4), cocaine (Non-Patent Document 5), or an ion (Non-Patent Document 6) changes its conformation by binding to a target. A method for measuring such a target, utilizing the above-described characteristics, has been developed (Non-Patent Document 7).
Using aptamers for affinity purification of, for example, a protein or the like is also attempted. According to this method, contaminations by substances derived from a protein such as a peptide can be extremely reduced as compared with a conventional method using antibodies. Therefore, according to this method, a purified product having a really high medical value and biochemical value can be obtained, for example (Non-Patent Document 8).
On the other hand, conventionally, a method for expressing a fusion protein obtained by fusing a peptide, as a tag, having several to dozens of consecutive amino acids with the N-terminal or the C-terminal of an intended protein in order to synthesize a large quantity of the intended protein has been known. According to this method, it is possible to check an expression of the intended protein and purify the intended protein, using the tag of the fusion protein as a clue. As the tag, a histidine peptide including several histidines, being called a histidine tag, is widely used, for example. A fusion protein to which the histidine tag has been added can be purified using a nickel ion column or an anti-histidine tag antibody, for example. However, there are problems in that the nickel ion column involves high nonspecific adsorption, and the anti-histidine antibody is expensive, and the like. Therefore, in the purification of an intended protein using the histidine tag, a development of a novel anti-histidine tag antibody or a binding molecule as an alternative of the anti-histidine tag antibody has been advanced.